1. Field of Invention
This invention relates generally to traffic monitoring systems, specifically to an ultrasonic traffic monitoring system.
2. Prior Art
The traffic pattern and density on highways and streets are constantly changing. In urban areas, congestion usually occurs during the early morning or late afternoon hours, i.e., the commute hours when people drive to and from work. Traffic slacks off somewhat during mid day, and considerably so at night. Traffic accidents can happen any time of the day, but most occur during the congested commute hours. Collisions often cause traffic to be backed up for miles and for several hours. Whenever traffic is congested, frequent stop-and-go driving is necessary and wastes a lot of fuel, and air pollution is substantially increased. Also the work force's productivity is reduced, because time spent in traffic is taken away from time spent at work. As our population and economy grow, our highways and streets will only get more crowded, and driving time will only get longer.
Many television and radio stations use observation helicopters for traffic reports. Drivers who are informed of the trouble spots by these reports can take alternate routes. However, a helicopter can take up to 30 minutes to circle a typical urban area, so that the situation at an area surveyed during the early part of a flight may be changed by the time the report is aired. As a result, such information sometimes become inaccurate by the time it reaches the commuters or drivers, so that they could be facing completely different situations on the roads. Also these reports are broadcast at predetermined and infrequent intervals. Most drivers depart without hearing a report, so that they could easily drive onto a congested highway or road instead of taking an alternative route. Another drawback is the vagueness of the reports, which usually describe the traffic as simply backed-up, heavy, or light, without information on the speed of traffic flow. The inaccuracy, infrequency, and vagueness of traffic reports make them of limited use.
Various electronic systems have been proposed for monitoring and reporting traffic conditions. These are usually based on radar and video technologies. A typical radar system uses sensors that are similar to police radar, and are mounted at selected locations on the highways. The speed data they gather may be relayed to a central station. The primary drawbacks of the radar systems include the high power consumption of the radar units, and the inaccuracy of their gathered data.
Video systems fall into two types that differ in their level of automation. In the first type, video cameras mounted close to highways deliver live pictures to a central station, via cable or microwave link, where human operators simply look at video monitors to mentally analyze traffic conditions. This type of system is already in use in several cities in the U.S. For example, SmartRoute Systems of Cambridge, Mass., employs this technology in the Boston area. In the second type of video system, each video camera is pointed at a single lane. Successive video frames are analyzed by a computer to determine traffic speed automatically. However, this system is expensive to produce and install. The cameras are vulnerable to theft because of their high value. Furthermore, their high power consumption prevents them from being self-powered by solar cells, so that they cannot be used in areas where electrical supply lines are not readily available. Still another drawback is their inability to function in poor weather, such as dense fog, snow fall, or blizzards.